One day as a young artist I was hours at the drawing board on a piece. After staring at it along time I realized something was wrong. I struggled with proportion for years as my skills developed. I picked the piece up and scanned it carefully. When I did I caught a glipse of it in a large mirror in my bedroom. I suddenly noticed the inconsistancies with the face and eyes. Looking back at the piece I wondered why I hadn't seen it before.
A fresh look was all I needed and the reversal of the mirror made it happen for me. Since then whenever I am disatisfied with a particular drawing I hold it up to the mirror and find my errors rather quickly.